lib/deploy: Throw an error if trying to stage when not ostree-booted

There's no reason to do this.  I didn't actually hit this problem,
but it's a corner case that just occurred to me while working on
the code.

I think callers should be adapted to skip trying to use staging
if there's no booted deployment.

Closes: #1568
Approved by: jlebon

tests/admin-test.sh

@@ -21,7 +21,7 @@
 
 set -euo pipefail
 
-echo "1..$((23 + ${extra_admin_tests:-0}))"
+echo "1..$((24 + ${extra_admin_tests:-0}))"
 
 function validate_bootloader() {
     cd ${test_tmpdir};
@@ -78,6 +78,12 @@ assert_ostree_deployment_refs 1/1/0
 ${CMD_PREFIX} ostree admin status
 echo "ok layout"
 
+if ${CMD_PREFIX} ostree admin deploy --stage --os=testos testos:testos/buildmaster/x86_64-runtime 2>err.txt; then
+    fatal "staged when not booted"
+fi
+assert_file_has_content_literal err.txt "Cannot stage a deployment when not currently booted into an OSTree system"
+echo "ok staging does not work when not booted"
+
 orig_mtime=$(stat -c '%.Y' sysroot/ostree/deploy)
 ${CMD_PREFIX} ostree admin deploy --os=testos testos:testos/buildmaster/x86_64-runtime
 new_mtime=$(stat -c '%.Y' sysroot/ostree/deploy)